Imagine if New York City schools were allowed to dismiss the worst-perform ing 6 percent of their teachers: How many tens of thousands of students would be saved from years of stunted educational growth?

Well, that’s exactly what just happened in Washington, DC.

Schools Chancellor Michelle Rhee yesterday handed pink slips to 241 public-school teachers — among them 165 who scored at the very bottom of the system’s new teacher-evaluation metric.

That’s more than one-20th of the 4,000 teachers in the entire system.

And another 737 teachers could face the ax next year unless they significantly improve their performance.

The purge is the result of a weakening of job-for-life tenure protections Rhee won last month in talks with the Washington Teachers Union — though the union now says it’ll contest the firings.

No doubt it will — that’s what factory-hand unions do. But teachers like to think of themselves as professionals — and professionals generally are held to account for the quality of their work.

Alas, not in New York.

Indeed, despite employing 20 times as many teachers as DC schools, the city’s Department of Education dismissed just five tenured teachers for incompetence in the 2008-09 school year. That’s not accountability — it’s a protection racket.

Sure, the DOE under reform-minded Chancellor Joel Klein is making commendable progress: Later this week, it expects to be releasing newly detailed teacher-evaluation data that might some day help weed out bad teachers.

But come this fall, even the worst of them will still be in a classroom.

Let’s face it: Compared with what Rhee’s achieved, Klein & Co. still have a very long way to go.